1. Field of the Invention
This invention relates to a method of hydrolyzing urea contained in waste water streams. In particular, the present invention relates to a novel way of treating waste water streams formed during urea production. Furthermore, the present invention relates to an improvement in the joint production of urea and ammonia.
2. Description of the Prior Art
Urea is commonly made today by reacting two moles of ammonia with one mole of carbon dioxide under elevated pressures and temperatures. This reaction forms one mole of urea and one mole of water as a by-product. The urea produced by this reaction is then separated from the reaction mixture by conventional concentration or crystallization methods. A solid urea product is thus recovered. The by-product water formed by this reaction forms one or more waste water streams. Such streams may contain a minor amount of urea and urea-related compounds, usually from about 0.05% to about 10% by weight. These streams also contain other impurities, such as unreacted ammonia and carbon dioxide.
In the past, these waste water streams were disposed of by dumping into the sewer. However, lately, it has been recognized by both the urea industry and the responsible governmental agencies that dumping urea-containing effluent streams may create possible environmental hazards.
Therefore, several processes have been suggested for treating these waste water streams. Some processes treat such streams in order to simply remove the ammonia and/or carbon dioxide contained therein, and leave any urea contained therein untouched. See U.S. Pat. Nos. 3,436,317, issued to Otsuka et al. on Apr. 1, 1969; 3,620,031, issued to Tennyson on Nov. 16, 1971; and 3,725,210, issued to Otsuka et al on Apr. 3, 1973. It is surmized that such references did not explicitly treat the urea in the waste water streams because urea is non-volatile in water and, therefore, is not easy to remove. In fact, the most effective way of removing the urea from the water is to hydrolyze it back to ammonia and carbon dioxide at above-ambient temperatures (the reverse reaction of the above-described urea synthesis). Some hydrolysis treatments have been disclosed. See U.S. Pat. Nos. 3,826,815, issued to Mavrovic on July 30, 1974 and 3,922,222, issued to Van Moorsel on Nov. 25, 1975. However, these methods of treating waste water containing urea have disadvantages. First, additional equipment must be constructed for these hydrolysis treatments. Also, this equipment must ordinarily be constructed of stainless steel since urea solutions may cause corrosion to ordinary carbon steel equipment. Therefore, the addition of such expensive equipment to the urea process would raise the cost of production. Furthermore, hydrolysis processes such as those illustrated in the cited patents require substantial energy costs because steam is normally required to heat the hydrolysis reaction.
At the present time, urea production plants are usually built near ammonia production plants because the two raw materials for urea production, namely, NH.sub.3 and CO.sub.2, can be readily obtained from the ammonia plants. The usual method for producing ammonia today is by the so-called synthetic gas route. According to this method, a synthetic gas mixture comprising hydrogen, nitrogen and carbon dioxide is formed. This may normally be done by any conventionally known processes for reforming natural gas and the like. After this synthetic gas mixture is formed, it is passed through a carbon dioxide recovery system whereby a major portion of the carbon dioxide in the gas mixture is separated out and recovered. The synthetic gas, now greatly depleted of CO.sub.2, is preferably then passed through a methanization step where any residual CO.sub.2 and carbon monoxide is changed into methane. Next, the N.sub.2 and H.sub.2 remaining in the gas mixture are converted into ammonia, usually in the presence of a catalyst. The CO.sub.2 and CO are normally removed from the synthetic gas mixture because they interfere with most conventional conversion catalysts, thereby lowering the yield of ammonia.
The carbon dioxide recovery system operation for such ammonia production processes normally comprises the steps of first passing the CO.sub.2 -containing synthetic gas through one or more absorption towers containing an aqueous solution. This aqueous solution is capable of absorbing a major portion of the CO.sub.2 from the synthetic gas without absorbing a significant amount of N.sub.2 or H.sub.2 and may contain one or more chemical additives (e.g., potassium carbonate) to enhance the CO.sub.2 absorption. After the CO.sub.2 has been absorbed into this aqueous solution, the solution is transferred to one or more stripping towers where the CO.sub.2 is stripped from the aqueous solution and then recovered. Preferably, after this stripping operation, the aqueous solution is recycled back to the absorption tower in order to absorb more CO.sub.2. The attached Drawing, discussed in detail below, illustrates one such CO.sub.2 recovery system. Further illustrations of carbon dioxide recovery systems are described in U.S. Pat. Nos. 3,851,041 and 3,896,212, both issued to Eickmeyer on Nov. 26, 1974 and July 22, 1975, respectively, and in U.S. Pat. Nos. 3,642,430; 3,685,960; and 3,823,222, all of which issued to Benson on Feb. 15, 1972, Aug. 22, 1972 and July 9, 1974, respectively. All five of these patents are incorporated by reference in their entirety.
Because urea and ammonia production facilities usually exist in close proximity to each other, it would be an advantageous solution to the above-mentioned waste water problem from the urea plant if the CO.sub.2 recovery system of the ammonia production process could be utilized without major modification to hydrolyze the urea contained in waste water streams formed in the urea production. In particular, the employment of a CO.sub.2 recovery system for this use could have several commercial advantages over existing hydrolysis treatments. One, since the CO.sub.2 recovery system already exists, no new costly equipment would have to be constructed. Further, since the CO.sub.2 recovery system normally operates at higher than ambient temperatures, hydrolysis of the urea can occur without the necessity of additional energy inputs (e.g., steam).